In previously used solutions in this category, the pistons in the piston assembly are placed inside the cylinder space opposite to each other, with the piston rods protruding from opposite ends of the cylinder, the length of each rod equalling essentially half the length of the cylinder space Thus the maximum length of the combination is essentially twice that of the cylinder space. In many cases, this is not sufficient in view of the required change in the length, especially when the space available imposes restrictions on the maximum length and other dimensions of the cylinder space. Such a situation prevails especially in telescopic boom structures which should be extendable to a sufficient length when working and still be able to contract into a sufficiently compact form when at rest.